| An ORNL group seeks to integrate various computer models of aspects of welding into one model
that will be useful to the welding industry. |
Predicting a Model Weld
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| Welding during construction of the Spallation Neutron Source at ORNL. (Photo by Jim Richmond, enhanced by Judy Neeley.) |
Land-based gas turbines for producing electricity will have turbine blades 1 m (36 in.) long that will be grown as a single crystal from a nickel-based superalloy. A single-crystal turbine blade may cost from $40,000 to $50,000. Thus, for cosmetic reasons or after long-term service, an urgent need may arise to repair the costly blade by welding it rather than to replace it. The goal of the repair would be a strong, tough, long-lasting weld. Certain questions must be answered, however, before the welding is done.
Should arc, laser beam, or electron beam welding be used? What should the composition of the weld material be? How fast and how long should the material be heated after welding, and what should the peak temperature be? Should the weld be rapidly or slowly cooled? Will the heating and cooling rates and compositional changes transform the microstructure of the weld to ensure it performs well?
Answers to all these questions are not readily available. However, an integrated computer model that would rapidly provide such answers for the welding industry is one goal of ORNL’s welding research group, which is supported by Department of Energy funding.
“Some researchers have developed
a process model that describes welding processes,” says Stan David, leader
of the ORNL group and a corporate fellow in ORNL’s Metals and Ceramics
(M&C) Division. “Others have prepared microstructure models that predict
microstructure and property changes based on alloy composition and heating
and cooling rates. Still others have written performance models that describe
how microstructural features affect the weld’s toughness, strength, ductility,
and final performance. We hope to combine all these models.”
M&C’s Suresh Babu, John Vitek,
Mike Santella, and others have developed kinetic and thermodynamic welding
models on a workstation. One model describes the role of oxide inclusions
in improving or degrading properties of steel welds. Oxygen from the air
that is dissolved in molten steel reacts with the steel’s residual elements
(e.g., aluminum, manganese, and titanium) to form oxide inclusions. Another
model can predict the right amount of aluminum to add to the steel-weld
material to prevent the degradation of its properties by oxygen and nitrogen
from the air.
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| ORNL’s
integrated welding process-microstructure-property computational model
can be used to design stronger, more reliable, energy-absorbing welds
needed in automotive fabrication. |
“We are one of the few welding groups in the world that use advanced thermodynamic and kinetic computational modeling to evaluate the evolution of microstructure in welds,” David says. But he also acknowledges that these models are only as good as the data they obtain from the characterization of weld samples before and after testing.
At ORNL weld specimens of various compositions (including new steels, nickel and iron aluminides, and carbon composites) are heated and cooled in a controlled manner by a computer-operated thermomechanical simulator. The spec-imens are stretched and compressed to test their strength and resistance to cracking. Before and after these tests, their microstructure is characterized using analytical electron microscopy, three-dimensional atom probe tomography, and neutron scattering at the High Flux Isotope Reactor (partly to measure residual stresses, which lead to cracking). The specimens are also studied using synchrotron X-ray scattering at the Stanford Synchrotron Radiation Laboratory and using neutron scattering at the ISIS Pulsed Neutron Source, Rutherford Appleton Laboratory, in the United Kingdom. All these studies allow the ORNL researchers to relate weld microstructure to properties and provide good data for their models.
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